After the reaction and optionally after removing the byproducts formed in the reaction, cellulose derivatives, in particular cellulose ethers, are present in friable, particulate or wad-like form. In this form the cellulose derivatives still have the structures predetermined by the raw materials. Thus for example cellulose ethers generally still exhibit the fibre-like structure of the initial cellulose. These cellulose derivatives are thus unsuitable for their intended use, for example as products soluble in organic and/or aqueous media.
It is also necessary to adjust specific grain size distributions, bulk densities, degree of dryness and viscosity values for the various areas of use.
In principle almost all cellulose derivatives therefore have to be compacted, ground and dried in order to make them fit for use.
The industrially important cellulose derivatives include in particular the cellulose ethers, whose production, properties and applications are scribed for example in: Ullmann's Encyclopaedia of Industrial Chemistry, 5th Edition (1986), Volume A5, pages 461-488, VCH Verlagsgesellschaft, Weinheim; and
Methoden der organischen Chemie (Methods of Organic Chemistry), 4th Edition (1987), Volume E20, Makromolekulare Stoffe, (Macromolecular Substances) Part-Volume 3, pages 2048-2076, Georg Thieme Verlag Stuttgart.
EP-A 0 049 815 describes a two-stage process for the production of micronised powders of cellulose ethers or cellulose, in which the products, which have a fine fibrous or woolly structure, are first of all converted into a brittle solidified form, and the thus treated material is subjected to a grinding operation until a grain size distribution of at least 90% below 0.125 mm is achieved. Vibratory mills or ball mills, preferably cooled, or pellet presses are used in the embrittlement stage, and jet mills, pinned disc mills or impact disc mills are used in the grinding stage.
EP-A-0 370 447 describes a process for the gentle grinding and simultaneous drying of moist cellulose ethers, in which a cellulose ether having an initial moisture content of 20 to 70 wt. % is conveyed by means of a carrier gas and at the same time is friction comminuted to an impact comminution size and is dried by the grinding energy to a residual moisture content of 1 to 10 wt. %.
EP-A-0 384 046 describes a process for the production of cellulose ether particles, comprising the comminution in a high rotational speed, air jet rotary impact mill to a particle size of 0.4 to 0.035 mm of cellulose ethers with a viscosity of greater than 100 Pa*s, measured as a 2% aqueous solution at 20° C. using an Ubbelohde tube.
EP-A-0 835 881 describes a process for the production of methyl cellulose powders with a special particle-size distribution curve by adjusting a methylhydroxyalkyl cellulose granular material to a moisture content of 30 to 70 wt. % with water, followed by grinding in a rotary mill, the product being simultaneously dried by the imparted grinding energy. A special particle-size distribution curve is established by size classification.
The known process according to the prior art are for the most part multi-stage processes involving a preliminary drier or preliminary embrittlement and/or compaction. Furthermore, in all the known processes the chemical and/or thermal stress on the macromolecules, particularly when processing extremely viscous, highly substituted products, is always also so intense that during the grinding the macromolecules are decomposed in the form of a chain scission, which is noticeable in particular by the more or less large decrease in viscosity compared to the starting products. Also the surfaces of the products treated by the preliminary embrittlement and/or preliminary drying steps become rough. Furthermore, a common feature of all the processes is the large amount of energy expended in grinding the cellulose derivatives after the preliminary drying, embrittlement or compaction.
Processes are already known that avoid one or more of the aforementioned disadvantages. For example, GB-A-2 262 527 describes a process for the comminution of cellulose derivatives by gelling a cellulose ether with a water content of 30 to 80 wt. % by cooling to a temperature of −10° to 60° C. followed by mill drying of the gel (preferably in an impact mill). In this process however air or nitrogen is used for the drying and conveying, which is then not recycled. The energy expended in carrying out the process is thus considerably higher than is theoretically necessary for evaporating the water contained in the ground material. Also, GB-A-2 262 527 does not give any indication of how to proceed with hydroxyethylated mixed ethers of methyl cellulose (such as for example methylhydroxyethyl cellulose or methylhydroxypropylhydroxyethyl cellulose) in order to obtain a product having a bulk density of greater than 300 g/l. The end product of the mill drying of methyl cellulose (Example 3) with a bulk density of 270 g/l is also not satisfactory.
WO 98/31710 describes a process for the production of finely particulate poly-saccharide derivatives by gelling or dissolving a cellulose ether with 35 to 99 wt. % (preferably 60 to 80 wt. %) of water referred to the total weight, followed by mill drying, wherein superheated steam is used to transport and dry the ground material. This process likewise avoids many of the aforementioned disadvantages. However, the comminution in this mill drying process is still not sufficient, and only 57 wt. % of the comminuted methylhydroxyethyl cellulose passes through a 0.063 mm sieve (Example 3). The desired fineness is achieved only by a size classification by means of an air jet sieve. Moreover the use of pure steam as carrier gas and heat exchange gas has disadvantages as regards the operational safety of the grinding plant since films and encrustations quickly form due to condensation of water together with the finely particulate polysaccharide derivative. The expenditure on maintenance measures is thus increased. Finely particulate products that pass in an amount of more than 95 wt. % through a 0.1 mm sieve or even in an amount of more than 90 wt. % through a 0.063 mm sieve are obtained according to this application only by dispersing dissolved polysaccharide derivatives in a non-dissolving ambient medium, followed by comminution, filtration and drying steps (Examples 1 and 2).
The object of the present invention is accordingly to provide a process for the production of particulate water-soluble cellulose derivatives, in which:                specific grinding levels can be set;        the surface of the products does not become rough; and        the bulk density of the products is increased.        